Niles states that while they were courting, he sent Maris a valentine stating:
You are the girl my heart adores, Everything I have is yours.
Now his lawyers are calling it a prenup. However, as we are all aware, Niles has stated (on more than one occasion) that there IS a prenup in place. (Take Season 4's "Are You Being Served?", for example).
Even though Maris might prefer to use the old valentine as a prenup, the laws of any state would preclude this because the prenup was clearly executed after the valentine. This means even if the valentine was deemed a valid prenup (which is unlikely), the existence of a later instrument that is final and complete (the actual prenup) would invalidate and override the earlier instrument.